Commercial crude terephthalic acid contains on a weight basis from 800 to 7,000 parts per million (ppm) 4-carboxybenzaldehyde 200 to 1,500 ppm p-toluic acid as the main impurities and some crude terephthalic acid also contain lesser amounts, 200- 20 ppm range, of yellow colored aromatic compounds having the structures of benzil, fluorenone or anthraquinone which are characteristically yellow compounds as impurities resulting from coupling side reactions occurring during the oxidation of p-xylene.
U.S. Pat. No. 3,584,039 issued to Delbert H. Meyer teaches a feasible, commercially useful method for purification of such commercially available crude terephthalic acid products by treating liquid phase solutions thereof in water at temperatures of 200.degree.-374.degree. C. with hydrogen in the presence of a solid hydrogenation catalyst (e.g., metallic palladium on carbon support) and crystallizing terephthalic acid from catalyst-free liquid phase solutions at temperatures in the range of 50.degree. to 150.degree. C. The catalytic hydrogen treatment converts 4-carboxybenzaldehyde to p-toluic acid and decolorizes the terephthalic acid.
British Pat. No. 1,152,575 is directed to the development of the Meyer Patent method for its commercial application by providing improved modes of conduct for the entire process from the step of dissolving crude terephthalic acid through the step of crystallizing terephthalic acid from the hydrogen treated aqueous solution. With respect to said crystallization, said British patent teaches the use of solvent evaporation to effect the cooling necessary to precipitate crystalline terephthalic acid but cautions that conduct of such evaporative cooling should avoid shock cooling of the solution as would occur by instantaneous flash evaporation of solvent because such shock cooling coprecipitates dissolved impurities which contaminate terephthalic acid product. To prevent the contaminating effect of such shock cooling, the British patent teaches that the evaporative cooling should be controlled by evaporation against equilibrium back pressure, for example, by throttling of steam vapor exhaust at the equilibrium pressure. This is in effect a controlled rate evaporative cooling.
Crystallization by controlled rate evaporative cooling is, according to the above British patent, applied to continuous crystallization conducted in three series connected stages under the conditions described to effect in 3.4 hours a 302.degree. F. temperature drop from 530.degree. F. initial solution temperature to the third stage temperature of 228.degree. F. This mode of conducting said crystallization provided an average cooling rate of 1.48.degree. F. per minute was not only inordinately slow but, when applied to aqueous solutions of terephthalic acid of 2,400 ppm p-toluic acid content, also provided a terephthalic acid product containing 1,200 ppm p-toluic acid. Such product would not be acceptable for direct reaction with ethylene glycol for polyester fiber manufacture.
U.S. Pat. No. 3,452,088 repeats the caution against shock cooling and teaches a further improvement for the continuous controlled rate evaporative cooling technique as applied to crystallizing terephthalic acid from aqueous solutions also containing dissolved p-toluic acid. The improvement consists of limiting the final crystallization temperature and/or crystalline product separation temperature to the temperature range of 250.degree. to 300.degree. F. to prevent p-toluic acid contamination of crystallizing terephthalic acid. By using such final crystallization and/or product separation temperatures of 250.degree. to 300.degree. F. terephthalic acid could be and was commercially obtained with 150 ppm and less p-toluic acid from feed solutions containing 6,000 to 500 ppm p-toluic acid at a somewhat faster cooling rate of 3.degree.-4.degree. F. per minute. But such faster controlled rate evaporation process does not provide a useful basis for devising still faster continuous flash evaporative crystallization to overcome the p-toluic acid contamination problem mentioned in both the British and U.S. patents.
Crystallization by flash evaporation of solvent has, in general, been long known and used to take advantage of the substantially instantaneous decrease in both temperature and pressure and attendant substantially instantaneous evaporation of solvent as the hot solution of solute is introduced into the crystallization vessel operated at a lower temperature and pressure. Advantageously, the rapidly vaporized portion of the liquid solvent flashed to the vapor phase permits rapid removal of solvent vapor. Both crystallization and crystal growth occur rapidly with the cooling and concentrating caused by flashing the solution to the lower temperature. Growth of crystals is substantially entirely the lower temperature and is independent of residence time. Crystal size in a crystallization vessel where solvent is flash evaporated can, as is well known, be enhanced by circulation of slurry of crystals throughout the lower portion of the crystallization vessel. For example, one means for accomplishing such circulation in a stirred crystallization zone is to withdraw a portion of the slurry from near its upper level and introduce, e.g., by pumping, the withdrawn slurry up through the bottom of the stirred slurry.
However, use of flash solvent evaporation induced crystallization of terephthalic acid (TA) from aqueous solution also containing dissolved p-toluic acid in amounts of 500 to 6,000 ppm based on TA can, without proper conduct thereof, bring into play the p-toluic acid contamination phenomenon alluded to in the British patent and more generally described in the later U.S. patent. Such contamination phenomenon is somewhat anomalous because, in spite of the fact that there is retained more than enough solvent water to prevent saturation or supersaturation with respect to p-toluic acid, p-toluic acid nevertheless comes out of solution. Said later U.S. patent suggests that the contamination phenomenon is in some way dependent on the rate of crystallization and the final temperature of crystallization and product separation and not solely on p-toluic acid concentration in the solution.
From plots of TA saturation and supersaturation (TA concentrations vs. temperature) and the guidance provided by teachings in the aforementioned related British and United States patents, one might devise a continuous TA crystallization process having a number of crystallization stages in series with each stage operated at a temperature lower than the preceding stage and, for smooth operation approximating batchwise crystallization, having a temperature profile substantially following the TA saturation plot. Such a devised continuous crystallization process would have at least about 40 rate dependent crystallization stages. However, because of the number of stages and their time consuming operation, such a continuous crystallization would not be economically attractive or feasible for commercial application.